The present invention relates to monitoring the processing of a substrate by reflecting light from features being processed on the substrate.
In the processing of substrates to fabricate electronic circuits and displays, semiconductor, dielectric, and conductor materials, are formed on a substrate by chemical vapor deposition (CVD), physical vapor deposition (PVD), oxidation, nitridation, ion implantation, and etching processes. In CVD processes, a reactive gas is used to deposit material on the substrate. In PVD processes, a target is sputtered to deposit material on the substrate. In oxidation and nitridation processes, an oxide or nitride material, such as silicon dioxide or silicon nitride, is formed on the substrate by exposing the substrate to a suitable gaseous environment. In ion implantation, ions are implanted into the substrate. In subsequent etching processes, etch-resistant features comprising resist or hard-mask, are formed on the substrate and the exposed portions of the substrate between the etch-resistant features (substrate open area) are etched to form patterns of gates, vias, contact holes or interconnect lines.
During such processing, a process monitoring method may be used to evaluate and control the process, for example, to stop or change the process at a process endpoint. In interferometric monitoring methods, interference peaks that result from the constructive and destructive interference of a radiation that is reflected from the substrate, over time, are counted to determine an etching depth into the substrate or a thickness of material deposited on the substrate. For example, in etching processes, as exposed portions of the substrate between etch-resistant features are etched to increasing depths, the intensity of the radiation reflected from the portions being etched and from underlying or adjacent material, modulates to form constructive and destructive interference peaks. By counting the interference peaks, the depth of the etched portions and the etching rate may be evaluated.
However, while such process monitoring methods can be used to determine vertical etching rates and etching depths, they are often not effective in evaluating other changing attributes of features being formed in the substrate. For example, conventional interferometric methods cannot be used to accurately determine changes in widths or other dimensions of the features being formed on the substrate, which are sometimes the critical dimensions. A change in a width of a feature may occur when sidewalls of the feature are being etched away or when material is deposited into the feature. While rate of etching of the feature depth is sometimes used to estimate the rate of etching of the sidewalls of the features, the two etching rates are often quite different, especially in processes in which the etching species have a kinetic energy component that enhances etch rates in the vertical direction relative to horizontal sidewall etching rates.
Thus, it is desirable to be able to monitor a changing shape or width of features being processed in a substrate, especially in the etching of features having critical dimensions or shaped profiles.